1. Field of Invention
This invention pertains to helical scan recording and reproduction of information from a storage medium, and particularly concerns the reading of non-native formats by helical scan recorder.
2. Related Art and Other Considerations
Numerous prior art patents and publications teach recording and reading of information stored in helical stripes (or "tracks") on magnetic tape. Examples of helical scan tape drives are shown, inter alia, in the following U.S. patents (all of which are incorporated herein by reference):
U.S. Pat. No. 4,835,628 to Hinz et al. PA1 U.S. Pat. No. 4,843,495 to Georgis et al. PA1 U.S. Pat. No. 5,065,261 to Hughes et al. PA1 U.S. Pat. No. 5,068,757 to Hughes et al. PA1 U.S. Pat. No. 5,142,422 to Zook et al.
In a helical scan arrangement, travelling magnetic tape is at least partially wrapped around a rotating drum so that heads (both write heads and read heads) positioned on the drum are contiguous to the drum as the drum is rotated. One or more write heads on the drum physically record data on the tape in a series of discrete stripes oriented at an angle with respect to the direction of tape travel. The data are formatted, prior to recording on the tape, to provide sufficient referencing information to enable later recovery during readout by one or more read heads.
Some helical scan systems utilize a guardband format wherein all tracks are recorded at the same azimuthal angle and each track or stripe is separated from its neighbors by a gap or guardband. See, for example, U.S. Pat. No. 4,843,495 to Georgis et al.
Other helical scan systems are dual azimuthal in nature, with only alternating tracks having the same azimuthal angle and with no guardband between tracks. See, for example, U.S. Pat. No. 5,065,261 to Hughes et al. and U.S. Pat. No. 5,068,757 to Hughes et al. This is done so read operations allow the read heads to overlap slightly into adjacent tracks. The azimuths are selected so as to filter out unwanted adjacent track read noise. Since the tracks are written and read in pairs, the heads within a pair are mechanically separated by a distance corresponding to one track pitch. The width of the other track of a pair is determined by tape speed relative to scanner speed. This mechanical separation of the read heads prevents the heads from reading formats of different track pitches by the usual servo tracking means.
Moreover, track pitch varies from helical scan system to another. As used herein, track or stripe "pitch" means a distance between centerlines of two adjacent tracks, the centerlines of the tracks extending along the direction of head travel and the distances therebetween being taken perpendicularly to the centerlines. In a dual azimuth system, track pitch equates to the width of a track.
In the above regard, the format disclosed in the Exabyte Corporation EXB-8200 device which is the subject of U.S. Pat. No. 4,835,628 to Hinz et al. employed a track pitch of 31 microns. A subsequent Exabyte Corporation EXB-8500 device utilized a track pitch of 15.5 microns. More recently there has been proposed the use of a track pitch of 10.75 microns (see U.S. patent application Ser. No. 08/150,726 filed Nov. 12, 1993 of Georgis and Zweighaft entitled "Method And Apparatus For Controlling Linear Tape Speed In A Helical Scan Recorder" (incorporated herein by reference).
A very important requirement for new tape drives is to read tapes written by previous generation drives. This requirement can extend to several generations of old formats. Most typically, as illustrated above, new drives write and read narrower tracks, so the write and read heads become correspondingly smaller. Eventually the read heads (or servo heads) become small enough to where they are not capable of reading servo information from both sides of a track, making conventional servo techniques impossible. Reading servo information from just one side of the track is possible, but provides limited performance for tracking acquisition.
Furthermore, most combinations of old formats on new drives do not lend well to a servo technique since it is not desired to have the head pass over the same point on each track, for the result would be missing data in other sections of each track. What is needed is a system which has the requisite head traversals for efficiently reading old (e.g., non-native) formats on new drives.